Abu Umaamah (RA) enquired from Nabi (Salla Allahu alayhi wa sallam),
"Which du'aa is most accepted by Almighty Allah?" Nabi (Salla Allahu alayhi wa sallam) replied, "The du'aa made in the last part of the night and the du'aa made after the fardh salaahs." (Tirmidhi vol. 2 pg. 187)
Anas (RA) narrates that Nabi (Salla Allahu alayhi wa sallam) mentioned, "Whichever of the servants of Allah spreads their hands after any of the Salaahs and say, 'O Allah, my Deity and the Deity of Ibrahim, Ishaaq, Ya'qoob, Jibreel, Mikaaeel and Israeefeel (alayhim salaam), I beg of you to accept my du'aa because verily I am hard pressed, and to protect me in my Deen because I am afflicted and to take me into Thy mercy because I am sinful and ward off poverty from me because verily I am needy' except that Allah takes it upon Himself that He will not return those hands empty handed, i.e. Allah will answer those du'aas. (ibnus Sunni pg. 61)
Ka'b Ahbaar (RA) mentions, "Verily it is mentioned in the Tawraat that Dawood (alayhis salaam) swears by Allah, The One who split the ocean for Moosa (alayis salaam) that Dawood (alayhis salaam) used to recite after salah, 'O Allah, rectify for me my Deen. That which you have made a protection for me and rectify for me my Dunya (worldly affairs) that in which I live, and I seek protection from your punishment through your pleasure, and I seek protection from your chastisement through your pardon, and I seek protection in you from you. None can withhold that which you provide and none can provide that which you withhold.
Suhayb (RA) narrates that Nabi (Salla Allahu alayhi wa sallam) used to mention the very same words upon the completion of salah. (Nasaaie; Tuhfatul Ahwazi vol.1 pg. 246)
Abu Ayub Ansari (RA) narrates, "Never did I perform salah behind Nabi (Salla Allahu alayhi wa sallam) but would here him upon the completion of salah say, 'O Allah, forgive my faults, sins and award me with the best of actions, and character. Verily none can guide to righteous actions nor can anyone cause one to turn away from evil actions but Thee.' (Mustadrak Haakim, Tuhfatul Ahwazi vol. 1 pg. 245)
Amr ibn Maymoon al-Awdi reports that Sa'd (RA) used to teach his son as a teacher would teach a child writing that Nabi (Salla Allahu alayhi wa sallam) used to seek refuge using the following words upon completion of salah, 'O Allah, I seek refuge in you from cowardice, from feebleness in old age, from the trials of the world and from the punishment of the grave.' (Bukhari vol. 1 pg. 396)
Mu'aadh (RA) narrates that Nabi (Salla Allahu alayhi wa sallam) advised me, 'O Mu'aadh, don't forget after every salah to recite 'O Allah, grant me strength upon your remembrance, gratitude and beautiful worship.'
Abu Hurayra (RA) narrates that Rasulullah (Salla Allahu alayhi wa sallam) used to recite after every salah, 'O Allah, I seek refuge in you from the punishment of the grave, from the punishment of the fire, from the trials of life and death and from the trials of Dajjal.' (Mustadrak vol. 1 pg. 237)
Muhammad ibn Yahya al-Aslami (RA) says, 'Once Abdullah ibn al-Zubayr (RA) saw a person lifting his hands in du'aa before completion from salah. When he completed his salah, Abdullah ibn al-Zubayr (RA) remarked, 'Nabi (Salla Allahu alayhi wa sallam) never lifted his hands (in du'aa) until he completed his salah.' (Tabrani vol. 13 pg. 129)
Habib ibn Salamah (RA) narrates that Rasulullah (Salla Allahu alayhi wa sallam) mentions, 'Never does a group gather, some calling and others begging acceptance, but Allah responds to that call.' (Kanzul Ummaal vol.1 pg. 177)
Salman (RA) mentions that Nabi (Salla Allahu alayhi wa sallam) has mentioned, 'Never do any people raise their palms asking Allah for something accept that Allah has taken it upon Himself to grant them that which they ask for.' (Majmauz-zawaaid vol. 10 pg. 172)
In one narration, it is stated, 'Verily, your Lord is living, kind and is shy to turn someone away empty handed when he lifts his hands.' (Abu Dawood)
From the abovementioned Ahadith, the virtue and significance of du'aa after salah is clearly evident. Furthermore, since the time after the Fardh salah is a time when one's du'aas are readily accepted, it will be permissible to make du'aa collectively.
However, it is not necessary to make du'aa after the fardh salah. If one regards making du'aa after the Fardh salah as compulsory, it will be an act of Bid'ah. To insist upon making du'aa after the fardh salah and to condemn one who does not make dua after Fardh salah is an indication of regarding that as compulsory. It is also not correct to stop anyone from making du'aa after Fardh salah, as that will be stopping someone from a permissible act, which is a sin.
and Allah Ta'ala Knows Best
Mufti Ebrahim Desai