Riddle: What is the sunnah that is better than an obligatory act?

The Shaykh : This is one of the matters that some scholars have posed as a riddle, saying: We have a sunnah that is better than an obligatory act. This is because it is agreed upon that the obligatory act is better; that is, the two rak'ahs of the Fajr prayer are better than its two rawatib, because it is obligatory. However, this sunnah is the initial greeting, and it is better than returning it, because returning it is based upon it.