It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "A man who purifies himself in his house and then goes out is counted as being in prayer until he completes his prayer; so let none of you interlace his fingers while he is in prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16371