It is narrated from Sayyiduna Wathilah bin al-Asqa' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "A man should not wipe his forehead until he has finished his prayer, but there is no harm in wiping the sweat from his temples, for the angels continue to invoke mercy upon him as long as the mark of prostration remains between his eyes."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18354