Musnad Ahmad 32:1315

'Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, from Humayd, from Anas, that the Prophet, peace and blessings of Allah be upon him, said while he was on his riding mount: "Labbayk (Here I am at Your service), there is no life except the life of the Hereafter, so forgive the Ansar and the Muhajirah (the Emigrants)," out of humility while on his mount.