Hedge fund mogul William Ackman has purchased a $3.3 billion stake in Valeant Pharmaceuticals International , making his firm the drug company's fifth-largest stakeholder, a source familiar with the matter said on Monday. Ackman's $20 billion Pershing Square Capital Management began building up the 5 percent stake in the Canadian drug maker this year, only months after his vocal effort to help Valeant buy Botox-maker Allergan failed.