Rashi on Leviticus 3:8/רש"י על ויקרא ג׳:ח׳

1וזרקו AND [AARON'S SONS] SHALL DASH THE BLOOD THEREOF [ROUND ABOUT] — two applications of the blood are necessary so as to constitute four (cf. Rashi on Leviticus 1:5). He (the priest) dashes the blood with a vessel (Zevachim 55a); it is only the blood of the sin-offering that he applies with his finger (Zevachim 53a).