How Do You Respond to Salam while Praying?
Question

If As-salam (the greeting of Islam) was said to someone during his prayer, what should he do?
Praise be to Allah.If someone greets you while you are praying , you may respond with a gesture. Suhayb (May Allah be please with him) said: "I passed by the Messenger of Allah (blessings and peace of Allah be upon him), while he was praying and I said Salam to him and he replied with a gesture ." 

The description of the gesture was narrated in many Hadiths, one of which is from Ibn `Umar (May Allah be pleased with him), who said: The Messenger of Allah (peace be upon him) went to Quba’ (first mosque in Madinah) to pray. He said that the Ansar (residents of Madinah) came to him and said Salam to him while he was praying . He said, "So, I said to Bilal, ‘How did you see the Messenger of Allah (peace be upon him) reply to them when they said Salam to him while he was praying?’" He said that he said, "Like this" and he flattened his hand. 
Jafar ibn Awn (one of the narrators) flattened his hand and made the bottom of the hand facing down and the back of the hand facing up. (From the book What Should You Do in the Following Situations...?”
And Allah knows best.