Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When you are praying and you have doubt between three and four (rak'ahs), and your strongest belief is four, recite the tashahhud, then perform two prostrations while sitting before giving the salam, then recite the tashahhud likewise, and then give the salam."
This is not strong, and there is disagreement over whether it is marfu' (attributed to the Prophet) and over its wording.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3901