Musannaf Ibn Abi Shayba 17:1364

Al-Fadl ibn Dukayn narrated to us, he said: Abu Hilal narrated to us, from Humayd ibn Hilal, he said: Abu Bakr spat and blew (lightly) to his right during an illness he was suffering from. He then said: "I did not do it except this one time," or he said: "other than this time."