Nafi' (may Allah have mercy on him) said that when Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was asked about forgetfulness in the prayer, he would say: Let one of you aim for what he thinks he has forgotten of his prayer and pray it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3811