Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 492
Narrated Suwaid bin An-Numan:
who was one of those who witnessed (the Pledge of allegiance beneath) the Tree: Allah's Apostle and his companions were given Sawiq and they chewed it.