Sayyiduna Ali (may Allah be pleased with him) said: The Sunnah in the obligatory prayer is that when one stands up after the first two rak'ahs, he should not support himself with his hands on the ground; however, if he is so aged that he cannot stand up straight without support, then it is permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2812