Hadith.999 - The Halabi asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man who forgot to say the Takbir (the opening Takbir) until he had entered the prayer.
Imam عليه السلام said: "Was it not in his intention to say the Takbir?"
He replied: "Yes."
Imam عليه السلام said: "Then let him continue with his prayer."