رَبِّ لَا تَذَرْنِي فَرْدًا وَأَنْتَ خَيْرُ الْوَارِثِينَ.
My Lord, do not leave me alone [with no heir], while you are the best of inheritors.
Rabbi la tadharni fardawwa Anta khayrulwarithin.
While Prophet Zakariyyah knew that Allah would convey the message of Tawheed to later generations, one way or another, he still wanted his own offspring to have a share in this honor and privilege. Hence, in his old age, and out of fear that he might die without being granted an heir to prophethood, he made this supplication and asked Allah for a child. Allah blessed him with Prophet Yahya who continued to carry the message to the Children of Israel. -Tafseer As-Sa’di and others.