311. When Amīr al-mu’minīn, peace be upon him, came to Baṣrah he sent Anas ibn Mālik to Ṭalḥah and az-Zubayr to make them recall what he (Anas) himself had heard the Messenger of Allāh, peace be upon him and his descendants, say concerning them both, but he avoided doing so and when he came back to Amīr al-mu’minīn, he said that he had forgotten that matter.
Thereupon, Amīr al-mu’minīn, peace be upon him, said: If you are speaking a lie Allāh may afflict you with white spots (leucoderma) which even the turban may not cover.
قال السيد الرضى: يعني البرص، فأصاب أَنَساً هذا الداء فيما بعدُ في وجهه، فكان لا يُرى إلاّ مُبَرقعاً.
as-Sayyid ar-Raḍī says: White spot means leucoderma. After sometime this disease did occur to Anas’s face so much so that he was never seen without a veil. 
 The occasion and circumstances surrounding this saying as related by as-Sayyid ar-Raḍī were that when at the time of the Battle of Jamal Amīr al-mu’minīn sent Anas ibn Mālik to Ṭalḥah and az-Zubayr with the purpose that he should recall them the Prophet’s saying to the effect that: “You two will fight ‘Alī and will commit excess over him”, he came back and stated that he had forgotten to mention it. Then, Amīr al-mu’minīn uttered these words about him. However, it is said that Amīr al-mu’minīn said the sentence on the occasion when he wanted Anas to confirm the Prophet’s saying: “Whosoever master I am, ‘Alī is his master. O’ my Allāh, love him who loves ‘Alī and hate him who hates ‘Alī”. Consequently, numerous persons have testified to this saying but Anas kept quiet. Then, Amīr al-mu’minīn, said to him, “You too were present at Ghadīr Khum, what is keeping you silent on this occasion?” and he said, “I have grown old and my memory does not serve me well”. Then, Amīr al-mu’minīn, pronounced this curse. (Ansāb al-ashrāf, al-Balādhurī, [concerning the biography of Amīr al-mu’minīn], pp. 156-157; al-A‘lāq an-nafīsah, Ibn Rustah, p. 221; Laṭā’if al-ma‘ārif, ath-Tha‘ālibī, pp. 105-106; Muḥāḍarāt al-udabā’, arRāghib, vol. 3, p. 293; Ibn Abi’l-Ḥadīd, vol. 4, p. 74; Arjaḥ al-maṭālib, ash-Shaykh ‘Ubaydullāh al-Ḥanafī, pp. 578, 579, 580).
In this connection, Ibn Qutaybah (‘Abdullāh ibn Muslim ad-Dinawarī
[213/828 — 276/889] writes : People have related that Amīr al-mu’minīn asked Anas ibn Mālik about the Prophet’s saying: “O’ my Allāh, love him who loves ‘Alī and hate him who hates ‘Alī,” and he replied, “I have grown old and I have forgotten it.” Then ‘Alī said: “If you are a liar, Allāh may afflict you with white spots which even the turban may not cover.” (al-Ma‘ārif, p. 580)
Ibn Abi’l-Ḥadīd has also supported this view and, denying the incident mentioned by as-Sayyid ar-Raḍī, writes : The incident mentioned by as-Sayyid ar-Raḍī that Amīr al-mu’minīn sent Anas ibn Mālik to Ṭalḥah and az-Zubayr is an unrecorded event. If Amīr al-mu’minīn had sent him particularly to recall to them the the Prophet’s saying concerning them, then he could hardly have come back and said that he had forgotten it because when he left Amīr al-mu’minīn and went to these two persons he should have admitted and remembered the saying, and therefore how could he, on his return after an hour or a day, plead that he had forgotten it and deny it. This is something that cannot happen. (Sharḥ Nahj al-balāghah, vol. 19, pp. 217 — 218)