Grading: Apart from the command to convey the salam, this hadith is "sahih", but this chain of it is "da'if" because of Muhammad bin Thabit al-Bunani.

Reference / Source: It was narrated by al-Tirmidhi (3903) with the chain Abdah bin Abdullah al-Khuza'i, from Abu Dawud (al-Tayalisi) and Abd al-Samad, and he declared it "hasan sahih" (in some copies: hasan gharib).

Details of the narration: Ahmad narrated it at 19/ (12521) through Abd al-Samad alone from Anas, but in this chain Abu Talhah (may Allah be pleased with him) is not mentioned.

Reference / Source: It was narrated by al-Ruyani (985), al-Khara'iti in "Makarim al-Akhlaq" (28), al-Shashi (1056, 1057), al-Tabarani in "al-Mu'jam al-Kabir" (4709) and al-Saydawi with the chain Muslim bin Ibrahim, from al-Hasan bin Abi Ja'far, from Thabit al-Bunani, from Anas, from Abu Talhah, as marfu': "May Allah reward you with good, O company of the Ansar, for as far as I know you are chaste and patient." But al-Hasan bin Abi Ja'far is a weak narrator.
Important point: Ibn Hibban (6264) narrated it as marfu' with the chain al-Zuhri, from Yazid bin Wadi'ah, from Abu Hurayrah: "The Ansar are chaste and patient", and this chain is "hasan" in corroborations and supporting reports.

Reference / Source: Ibn Abi Shaybah 12/ 160 narrated it by way of Muhammad bin Ishaq, from Asim bin Umar bin Qatadah, as "mursal", that whenever the Prophet (peace and blessings be upon him) mentioned the Ansar he would say: "(They are) chaste and patient."

Note / Clarification: See also the narration after it.