How authentic is the statement attributed to Umar ibn Al-Khattab (may Allah be pleased with him) regarding the acceptance of supplication when sending prayers upon the Prophet (peace and blessings be upon him) before and after it?

It was not reported from Umar (may Allah be pleased with him) that Allah accepts supplications if they begin and end with prayers upon the Prophet (peace and blessings be upon him). Rather, this was reported from Abu Sulayman al-Darani. Scholars have mentioned the desirability of beginning and ending supplications with prayers upon the Prophet (peace and blessings be upon him) because they are accepted, and Allah is too generous to reject what lies between two accepted acts. This has been affirmed in the noble Prophetic Sunnah, such as in the hadith of Fadalah ibn Ubayd and others.