Abu Thumamah al-Hannat said: Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) met me while I was heading to the mosque with my fingers interlaced, and he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When one of you performs ablution and then comes to the mosque, let him not interlace his fingers, for he is in a state of prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16382
Abu Thumamah said: I met Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) on a Friday while I was going to the Friday prayer. When I saw him I hurried towards him, and I had interlaced my fingers. When I came near him, he struck my hands and separated my fingers. I said: O Abu Muhammad, what is this? He said: We have been forbidden from any of us interlacing his fingers in prayer. I said: But I am not in prayer. He said: Have you not performed ablution while intending the Friday prayer? I said: Of course. He said: Then you are in a state of prayer.
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16383