It's from shaytan.
Al-Bukhaari (6226) and Muslim (2994) narrated from Abu Hurayrah (r) that the Prophet (saaws) said: “Allah likes sneezing and dislikes yawning, so if one of you sneezes and praises Allah (by saying Al-hamdu Lillaah), it is a duty upon every Muslim who hears him to say to him, Yarhamuk Allah (may Allah have mercy on you). With regard to yawning, it is from the Shaytan so if one of you feels like yawning, let him suppress it as much as he can, for if one of you yawns the Shaytaan laughs at him.
Al-Manawi(r) said: It is attributed to him because he is the one who calls for giving the self its share of desires. What is meant here is to warn against the things that lead to that, which is eating too much and eating one's fill, which makes the body feel too heavy and tired to do acts of worship.