Musnad Abu Ya'la al-Mawsili 129:118

Abu Khaythama narrated to us, Yazid narrated to us, Humayd al-Tawil informed us, from Anas, that Abu Musa sought a mount from the Prophet, may Allah bless him and grant him peace. He found him occupied, so he (the Prophet) swore, "I will not mount you." He (Anas) said: And I swear, I will mount you. So he mounted him.