Musannaf Ibn Abi Shayba 3:6096

Yazid ibn Harun narrated to us, from Juwaybir, from al-Dahhak, who said: If he sits after the completion of the prayer, and he breaks wind before he performs the tashahhud, or after the tashahhud before the Imam performs the taslim, then it is valid, so let him depart.