Walaikum assalam wa rahmatullah,

Yes, it is rigorously authenticated (sahih).

Imam al-Ajluni (Allah have mercy on him) mentioned the hadith,

“Say the truth, even if bitter,” was related by Ahmad from Abu Dharr (Allah be pleased with him) from the Messenger of Allah (Allah bless him and give him peace).

It has supporting hadiths, such as what Bayhaqi from Jabir (Allah be pleased with him) from the Messenger of Allah (Allah bless him and give him peace) with the [similar] words, “There is no charity more beloved to Allah from speaking the truth,” and this was declared rigorously authentic (sahih) by Ibn Hibban.

Wassalam,

Faraz Rabbani

http://qa.sunnipath.com/issue_view.asp?HD=7&ID=2285&CATE=120