SHAFAQNA -It is narrated from Imam Reza (AS) who said: The prostration (Sijdah) after performing Wajib Salaat, is a kind of thanking God for the blessing granted to God’s servant to be able to perform his/her religious duty, and the saying which is repeated at least three times while prostrating is: “Shokran Lellah”. Imam Reza (AS) was asked: What is the meaning of “Shokran Lellah”? Imam Reza (AS) replied: It means that the one who is prostrating is saying: I am prostrating to thank God for the opportunity granted to me to serve God, and to perform my religious duty, and giving thanks to God will add to (my) good deed. And if there are any shortages or deficiencies in Salaat, which are not compensated for by performing recommended Salaats (Nawafel), with this thanksgiving prostration, those deficiencies will be removed, and the Salaat is completed .
 Mosnadul Imamel Reza, Vol. 2, page 31.