Follow-up thread: Does the angel in Q. 19:19 say to Mary "I shall give" or "He[God] shall give" her a pure son? Both readings are attested. The Andalusian Abū Bakr al-Zubaydī (d. 989) nicely captures the conundrum in an anecdote from his Ṭabaqāt al-naḥwiyyīn wa-l-lughawiyyīn...
The story is about Muḥammad ibn Abī Muḥammad al-Yazīd, a qurʾan-scholar and tutor to al-Maʾmūn (r. 813–833) at the Abbasid court. The story is recounted by the scholar’s brother, Ibrāhīm, who narrates it as follows ... 
My brother Muḥammad ibn Abī Muḥammad used to instruct al-Maʾmūn in qurʾan recitation daily, but when he tired of my brother’s instruction, al-Maʾmūn said to him, “Muḥammad, it is too burdensome for me to recite the Qurʾan to you because ...
I am forced to raise my voice more than I can manage. [He was hard of hearing. -SWA] Order your brother Ibrāhīm and your son Aḥmad”—that is, Abū Jaʿfar—“to come one day so that I may read to him. You will be present, and if I have any doubts arise, I will ask you about it.” ...
[Ibrāhīm continued:] When it was the day of my turn [al-Maʾmūn] recited Sūrah Maryam to me, and he said, «I am sent by your Lord so that He will grant you … (innamā anā rasūlu rabbiki li-yahaba laki)» (Q. 19:19)...
Yaḥyā ibn Aktham then said, “Commander of the Faithful, I prefer for you not to read this recitation (hādhihi l-qirāʾah)!”
“And why not?” al-Maʾmūn asked.
“Because,” Yaḥyā said, “it contravenes the muṣḥaf.”
Al-Maʾmūn then turned to me and said, “What say you, Ibrāhīm?”
“And why not?” al-Maʾmūn asked.
“Because,” Yaḥyā said, “it contravenes the muṣḥaf.”
Al-Maʾmūn then turned to me and said, “What say you, Ibrāhīm?”
I replied, “Commander of the Faithful, this is a reading recited by quite a number of the Companions of the Prophet, the first of whom was your very ancestor ʿAbdallāh ibn al-ʿAbbās!”
My brother Muḥammad then turned to me and said, “What are you all discussing, Ibrahīm?”
My brother Muḥammad then turned to me and said, “What are you all discussing, Ibrahīm?”
I replied, “The Commander of the Faithful recited, «I am sent by your Lord so that He will grant you … (innamā anā rasūlu rabbiki li-yahaba laki)» (Q. 19:19), and Yaḥyā said, ‘I prefer for you not to read this recitation (hādhā l-ḥarf).’ And when he asked why, he said,
‘because it contravenes the muṣḥaf’.”
My brother then said to al-Maʾmūn, “Of what concern is this to Yaḥyā! This is a reading (ḥarf) that many of the Companions of the Prophet recited, as well as their followers! Is whatever is in the muṣḥaf recited?
My brother then said to al-Maʾmūn, “Of what concern is this to Yaḥyā! This is a reading (ḥarf) that many of the Companions of the Prophet recited, as well as their followers! Is whatever is in the muṣḥaf recited?
By God, Commander of the Faithful, were it not recited that God informed us that the angel came to her and said “I am sent by your Lord so that He will grant you”—and not, “so that I will grant you”—then it would necessary for it to be read thus!”
Yaḥyā spoke not a word more.
Yaḥyā spoke not a word more.
