From Annal 15:44
The following is a public domain translation of the above Latin text:
Some scholars believe this could be a later textual interpolation by Christian scribes. Unlike the case with Josephus on Jesus, however, there is no clear evidence for doubting the authenticity of this text.
Tacitus is considered the most reliable scholar of his time. He had access to Roman archives, and his only mistakes arose from occasional reliance on secondary sources. In this case he could have been using either Christian sources or Roman archives. It is argued that if he had been using Roman archives, he should have identified Pontius Pilate as a "prefect" rather than a "procurator," but that is disputable. The more serious criticism is that the records would have identified Jesus by his given name rather than "Christus." Although Tacitus was Roman rather than Jewish and might have believed that was part of the name, it is extremely unlikely he would have selected it alone from the archives. In addition, Christian accounts were readily available while centuries of inquiry have turned up no Roman documents related to a historical Jesus. The conclusion is that the information must have been derived from Christian sources. Thus it offers no independent evidence for the existence of a historical Jesus.
Regarding the reliability of Tacitus, the Catholic Encyclopedia mentions "the credulity with which he accepted the absurd legends and calumnies about the origin of he Hebrew people (Hist., V, iii, iv)." (http://www.newadvent.org/cathen/08375a.htm)