It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) came out to us and said: "How excellent are those who do khilal!" The people said: O Messenger of Allah, who are those who do khilal? He (peace and blessings be upon him) said: "Those who do khilal in ablution, and those who do khilal after eating. The khilal of ablution is: rinsing the mouth, sniffing water into the nose, and (making water reach) between the fingers. The khilal of food is removing (particles of) food (from the teeth), for nothing is harder on the two angels than seeing something between the teeth of their companion while he is standing in prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4046