This claim comes from about.com. It references this source which doesn't claim that Porus specifically did it, merely that such a practice was common. It also cites Adrienne Mayor who claims that Alexander's men encountered poisoned arrow heads.
A separate source on the University of Washington website claims that:
Diodorus describes Alexander’s Army encountering war elephants of Indian King Porus and the siege of Harmatelia (todays SW Pakistan) in 326 BC. The warriors had smeared their spears swords and arrows with snake venom from Vipera russelli.
The wounded Greeks went numb, exper convulsions, vomited bile and a purple-green gangrene to horr. death. After a plant was found as antidote “by Alex” the Greeks defeated the barbarians.
- Were the swords/elephant tusks/arrows of Porus's army poisoned?
- What was this antidote that Alexander found?
EDIT: It seems that about.com misinterpreted Adrienne Mayor's work, see her comment below.