It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Any man who performs ablution in his house and then goes out intending the prayer is counted as being in prayer until he completes his prayer; so let him not interlace his fingers while in prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16387