Sheikh Sa`d al-Shuwayrikh
I have never come across any reference in the Sunnah to support the practice of saying “A`ûdhu bil-lâh min al-Shaytân al-Rajîm” (I seek refuge with Allah from Satan the Accursed) after yawning.
However, it is related in more than one authentic hadîth that yawning is from Satan and that we should stifle our yawns as much as possible.
The Prophet (peace be upon him) said: “Yawning is from Satan. If any of you yawns, let him stifle it as much as possible.” [Sahîh al-Bukhârî and Sahîh Muslim]
In another narration, he said: “If the one of you says ‘haa’, Satan will laugh at him.” [Sahîh al-Bukhârî]
It is also related that Allah likes sneezing and dislikes yawning. The reason why yawning is disliked is that yawning usually indicates laziness and an over-full stomach.
And Allah knows best.
Source: Islam Today