Admonition for leaving out a Sunnah
Extract from Sheikh Mahmoodul Hasan Life and Works (pg 119)

Just as Hadhrat (RA) expressed happiness when he saw someone practicing a Sunnah, he also became very upset if he saw any of his associates contradicting a Sunnah. Once, on the dastarkhaan, an ‘aalim, who was a student of Hadhrat (RA) needed to drink water. He took the glass in his left hand and placing his right hand under the glass started drinking the water. Hadhrat (RA) was very upset with him and addressed him firmly, saying, “Don’t you have a right hand?” He replied, “Hadhrat, my right hand was messed with food, and I did not wish to dirty the glass.” Hadhrat (RA) angrily said to him, “Is this glass more valuable than the Sunnah?” Thereafter he demonstrated the method of holding the glass by saying, “Lick your fingers after eating and       hold the bottom of the glass by making a circle with your shahadah finger and the thumb.”