Amr bin Dinar (may Allah have mercy on him) narrates that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: "Whoever has offered the obligatory prayer and then wishes to offer voluntary prayer after it, let him move a little forward or back, or talk with someone."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3050