One should avoid yawning. This is from the shaytan (devil). With the help of yawning, the shaytan will increase you in sleep and encourage you to become tired, and you will unable to do your day to day activity to the best possible.
- It is narrated on the authority of Abu Hurayrah  that he said The Messenger of Allah  said: "Yawning is from the Devil. If one of you yawns let him suppress it as much as possible, as when one of you goes ahhh, the Devil laughs."